The only factory seats that are going to be a direct bolt in would be from the 90-95 Runer, or an '89-95(pre Taco) truck that was an SR5 model. Tacoma and 3rd generation -Runner seats should also fit with little to no modification. Most of the other Toyota buckets will fit and usually 2-4 of the existing bolt holes will match up with the bucket seat brackets. I run a set of '94 Supra seats in my '91, the front mounts lined up, the rear outer mounts lined up but the seat would have sat to low, so I had to weld up a bracket that lifted the rear of the seat so it would be at the same hieght as the stock 60/40. Also with most car buckets you have to lift the outer bracket a little to get the seat back to be level. It doesn't effect the ride comfort at all just looks better with the backs being level.
You migth also consider leaving the 40 seat in and picking up another 40 seat at a wrecking yard. The hinges and seat back lever from the 60 section should swap over to the 40 section. If you aren't in a hurry you should be able to find one in a matching color, grey and blue were a common color for those seats.
I would really prefer some that I can use with the standard seat belt so my kid can ride along with me
If you mean the existing main seat belts(outer sets), with either set-up the seat belt on the passenger side will remain the same, it is alraedy set-up for a bucket seat. The drivers side you will want to relocate it to the tranny tunnel, like the way it is on the passenger side. If yoiu are needing to carry 3 passengers the rear of the 2nd gen('89-95) X-cabs came with seats and seat belts. The X-cabs w/60/40 seat could carry 5 passengers all with their own seatbelt.
When looking for seats you should start with all Toyota trucks and 4Runners (pre 2nd gen Tacoma's and 4th Gen $-Runner), then look at Toyota sport cars, Celica, Supra, Mr2, and the Paseo. Camry and Corolla seats would be next, or other import sport cars like 300z's Rx-7's, or Honda Prelude, last seats I would look into would be from import cars, since a lot of the car buckets are taller and tend to reduce headroom.